1. Field of the Invention
The present invention relates, in general, to image forming apparatus. In particular, the invention relates to an image forming apparatus which uses electrostatic photography technology to form a visible image corresponding to an original document on a paper.
2. Description of the Related Art
In a conventional image forming apparatus which forms an image using an electrostatic photography technology, the information recorded surface of a document is scanned by a light beam and the light reflected from the information recorded surface is focused on the outer surface of an image carrier, e.g., a photosensitive drum, to form an electrostatic latent image on the image carrier. The electrostatic latent image formed on the surface of the image carrier is developed by the electrostatic adhesion of a developing agent to be a visible image.
In general, one of two different developing agents is used to develop the electrostatic latent image formed on the image carrier. One developing agent may be a two component developing agent, in which thermoplastic colored toner particles are mixed with carrier particles such as ferromagnetic particles. The other developing agent may be a single component developing agent in which each toner particle is integrated with a carrier particle.
The two component developing agent has a good environment-resisting ability, e.g., image density against high temperature or high humidity. This is because a high electrostatic charge can be produced by the mechanical friction between the toner particles and the carrier particles, in spite of high humidity. On the other hand, the one component developing agent has a good maintenance record during the operation of the image forming apparatus because of the integration of toner particles and carrier particles.
However, if the two component developing agent is used, it is necessary to maintain the mixture ratio between the carrier particles and the toner particles at a constant value to produce images of a constant image density. Thus, an auto-toner supply system is employed if the two component developing agent is used.
In the auto-toner supply system, a toner senser is used to detect the toner density of the two component developing agent stored in the developing device. When a low toner density is detected by the toner senser, the toner is replenished in the developing device.
In the conventional image forming apparatus employing the above-described auto toner supply system, the toner of the two component developing agent is given a prescribed electric charge against the electrostatic latent image formed on the surface of the image carrier. The toner of the two component developing agent is attached to the electrostatic latent image on the image carrier, as a toner image, when the electrostatic latent image is developed, and the toner image is transferred to an image forming medium, e.g., a paper sheet, in a transferring process. Thus, the toner in the two component developing agent is consumed every time the developing process is carried out.
To decrease the consumption of the toner described above, the toner which remains on the image carrier after the transferring process is completed is collected and returned to the developing device. Such a toner collecting operation is called as a toner recycling system.
In the above-described toner recycling system of the conventional image forming apparatus, if an image having a low image forming rate (area of images formed on the image forming medium/printable area of the image forming mediums) is repeatedly printed or copied, untransferred toner particles on the image carrier are collected on the developing device, and the collected toner particles are reused repeatedly. If envelopes are used as an image forming medium to be printed by the above-described image forming apparatus, the image forming rate of each envelope is low, e.g., 1.about.2% (where as the image forming rate of a paper document is about 5%). This is because, in general, only an address is printed on the envelope. Thus, the above-described toner particles are repeatedly collected and stirred in the developing device resulting in fine toner particles. Since fine toner particles gather to make a mass or group, the volume thereof is increased although the mass is the same as that in the original state, as a whole. Thus, the amount of two component developing agent which contacts the toner sensing unit is increased, resulting in an increased output of the toner sensing unit. The increase in the output of the toner sensing unit falsely indicates the same result as the decrease of the toner density of the two component developing agent stored in the developing device. Responding to this false result, unused toner is replenished to the developing device and the actual toner density in the developing deive becomes excessively high. The background of the image formed on the image forming medium, e.g., a paper, tends to be black, and thus, a fogged image may be formed on the image forming medium.
As described above, if a document having a low image forming rate, such as, e.g., an envelope, is continuously copied by the conventional image forming apparatus, an average toner particle size in the developing device tends to decrease. A particle size of the toner having a diameter of about 11 .mu.m at an initial stage is decreased up to 9 .mu.m (about 20% decrease) after 15,000 envelopes are copied. Then, the volume of the toner in the developing device is increased, in contrast with the decrease in an average particle size of the toner, and thus, the output of the toner sensing unit is increased to replenish unused toner into the developing device. As a result, a fogged image may be formed on the envelope. Furthermore, if the average particle size of the toner becomes smaller, the image formed on the document tends to be fogged.